Multi-Spectral Super-Pixel Filters and Methods of Formation

ABSTRACT

Multi-spectral filter elements and methods of formation are disclosed. Each multi-spectral filter element may include a plurality of sub-filters that are, in some embodiments, each adapted to respond to electromagnetic radiation within respective ones of a plurality of spectral bands. A method embodiment includes forming an optical cavity layer. Volume of the optical cavity layer can be reduced in at least N−1 number of spatial regions. The reducing may include a number of selective removal steps equal to the binary logarithm function Log 2  N. In this example, each spatial region corresponds to a respective one of the plurality sub-filters. The plurality of sub-filters include at least N sub-filters. In particular embodiments, the respective ones of the plurality of spectral bands may be at least partially discrete with respect to each other.

TECHNICAL FIELD

This invention relates generally to radiation filtration, and moreparticularly to multi-spectral super-pixel filters and methods offormation.

BACKGROUND

Current multi-spectral imaging systems are limited for a variety ofreasons. For example, many multi-spectral imaging systems are bulky andcostly due to the fact that they typically rely on optics elements toseparate the spectral information. Many systems are either filter-wheeltype or diffractive/refractive type, resulting in the need for manyframes to make one multi-spectral cube. Some systems are one-dimensionaland rely on scanning architecture, thereby further increasing cost.These and a variety of other limitations often greatly limit infrared(IR) spectroscopic applications.

SUMMARY OF THE INVENTION

Multi-spectral filter elements and methods of formation are disclosed.Each multi-spectral filter element may include a plurality ofsub-filters that are, in some embodiments, each responsive toelectromagnetic radiation within respective ones of a plurality ofspectral bands. A method embodiment includes forming an optical cavitylayer. Volume of the optical cavity layer can be reduced in at least N−1number of spatial regions. The reducing may include a number ofselective removal steps equal to the binary logarithm function Log₂ N.In this example, each spatial region corresponds to a respective one ofthe plurality sub-filters. The plurality of sub-filters include at leastN sub-filters. In particular embodiments, the respective ones of theplurality of spectral bands may be at least partially discrete withrespect to each other.

In one embodiment an array of multi-spectral filter elements comprises aplurality of multi-spectral filter elements, where each of the pluralityof multi-spectral filter elements is adapted to respond toelectromagnetic radiation within a plurality of spectral bands. In thisembodiment, each of the plurality of multi-spectral filter elementscomprises a first sub-filter element that is adapted to respond toelectromagnetic radiation within a first one of the plurality ofspectral bands. The first sub-filter element comprises a first opticalcavity having an average thickness of approximately X. Each of theplurality of multi-spectral filter elements comprises a secondsub-filter element that is adapted to respond to electromagneticradiation within a second one of the plurality of spectral bands. Inthis example, the second sub-filter element comprises a second opticalcavity having an average thickness of approximately X−Y. Each of theplurality of multi-spectral filter elements further comprises a thirdsub-filter element that is adapted to respond to electromagneticradiation within a third one of the plurality of spectral bands. Thethird sub-filter element comprises a third optical cavity having anaverage thickness of approximately X−Z. In addition, each of theplurality of multi-spectral filter elements comprises a fourthsub-filter element adapted to respond to electromagnetic radiationwithin a fourth one of the plurality of spectral bands. The fourthsub-filter element comprises a fourth optical cavity having an averagethickness of approximately X−Y−Z. In this particular embodiment, each ofthe first, second, third, and fourth optical cavities compriserespective portions of a solid dielectric layer.

Particular embodiments disclosed herein may provide one or moretechnical advantages. For example, various embodiments may enable alow-cost, multi-spectral sensors having reduced system size andcomplexity. Various embodiments may be capable of spatially filtering anN number of spectral regions using N sub-pixels filters. The cavity ofthese N sub-pixels filters may be shaped to respective dimensionssuitable for desired resonation using as few removal steps of the cavityas the binary logarithmic function Log₂ N. Certain embodiments mayimprove device performance by mitigating harmful diffraction. Particularembodiments may enable a denser pixel arrangement that facilitateshigher resolution. Particular embodiments may enable the formation oflayers-in-common for discretely tuned optical filters. Certainembodiments may provide all, some, or none of these advantages. Certainembodiments may provide one or more other advantages, one or more ofwhich may be apparent to those skilled in the art from the figures,descriptions, and claims included herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention andadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings, in which:

FIGS. 1A through 1C illustrate one example embodiment of a portion of amulti-spectral filter at various stages of formation including one ormore super-pixels according to one embodiment; and

FIG. 2 illustrates a cross-section of one example embodiment of asub-pixel that may be used to faun a portion of the super-pixel of FIGS.1A through 1C.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Various example embodiments disclosed herein are explained in thecontext of multi-band radiation filters and methods of formation.Certain embodiments may provide enhanced spectral imaging performancewithin multiple spectral regions. Additionally, certain embodiments maybe formed using precise and relatively inexpensive semiconductorprocessing techniques. Although various example embodiments disclosedherein are explained in the context of filtering light provided to aninfrared focal-plane (IR-FPA), the teachings of the present disclosurecould be applied to any of a variety of alternative applicationsincluding, for example, photodiodes, photoconductive detectors,photovoltaic detectors, photodiode detectors, or any other suitableradiation filter and/or detector responsive to a variety of differentspectral regions. Additionally, particular embodiments disclosed hereinmay be implemented using any number of techniques, whether currentlyknown or in existence. The present disclosure should in no way belimited to the example implementations, drawings, and techniquesillustrated below. The drawings are not necessarily drawn to scale.

FIGS. 1A through 1C illustrate one example embodiment of a portion of amulti-spectral filter system 100 including one or more super-pixels 102at various stages of formation. In particular embodiments, filter system100 may be a color filter array (CFA) configured to filter lightprovided to a surface of focal-plane array (FPA), such as, for example,an IR-FPA. In this regard, system 100 may function as a Bayer filterhaving an array of multiple super-pixels 102 arranged in a one-, two-,or three-dimensional pattern. Each super-pixel 102 may have two or moresub-pixel filters (e.g., sub-pixel filters A, B, C, and/or D) that areresponsive to electromagnetic radiation within respective ones of aplurality of differing spectral bands. In particular embodiments,multi-spectral filter system 100 may be configured to be coupled to afocal-plane array, such as, for example, an IR-FPA. Each super-pixel 102of system 100 may be aligned to, and may be configured to filter lightprovided to, one or more corresponding pixels of the IR-FPA.

Super-pixel 102 generally includes a substrate 104, one or morereflective layers 106, and an optical cavity 108. Although particularembodiments may include a substrate 104, one or more reflective layers106, and an optical cavity 108, alternative embodiments may include all,some, or none of these layers. Additionally, alternative embodiments mayinclude any suitable number of additional and/or alternative layersincluding, for example, one or more interstitial layers that may or maynot be capable of filtering, absorbing, and/or transmitting radiation.

As explained further below, portions of optical cavity 108 may beselectively removed, such that optical cavity 108 has a differentrespective thickness for each sub-pixels filters A, B, C, and/or D. Thevarying thicknesses of optical cavity 108 may cause sub-pixels filtersA, B, C, and/or D to resonate at discrete optical wavelengths. Theresonant properties of sub-pixels filters A, B, C, and/or D may resultin a multi-spectral super-pixel 102 capable of filtering and/orabsorbing radiation in multiple discrete bands (four in this example).Although super-pixel 102 includes four sub-pixels filters A, B, C, and Din this example, super-pixel 102 may include fewer or more sub-pixelsfilters including, for example, one, two, ten, fifty, hundreds, or moresub-pixels filters.

FIG. 1B illustrates relative thicknesses of sub-pixel filters A, B, Cand D after optical cavity 108 is reduced by a thickness ofapproximately y in selective areas. In this example, the selectivelyremoved portions of optical cavity 108 overlay sub-pixel filters B andD. The thickness of optical cavity 108 overlaying sub-pixel filters Aand C are substantially unchanged with respect to the correspondingthicknesses illustrated in FIG. 1A. In relative terms, optical cavity108 has an initial average thickness of approximately x and theselectively removed volume has a thickness of approximately y. Anysuitable processes may be used to selectively remove portions of opticalcavity 108 including, for example, photolithographic patterning andetching.

FIG. 1C illustrates relative thickness of sub-pixel filters A, B, C, andD after optical cavity 108 is reduced by a thickness of approximately zin selective areas. In this example, the selectively removed portions ofoptical cavity 108 overlay sub-pixel filters C and D. The thickness ofoptical cavity 108 overlaying sub-pixel filters A and C aresubstantially unchanged with respect to the corresponding thicknessesillustrated in FIG. 1B. As a result of the selective removals shown inFIGS. 1B and 1C, sub-pixel filters A, B, C, and D have average opticalcavity 108 thicknesses of approximately x, x−y, x−z, and x−(y+z),respectively. These thicknesses x, x−y, x−z, and x−(y+z) may eachcorrespond to particular wavelength resonation. In this example, fourdifferent sub-pixel filters A, B, C, and D are formed with varyingoptical cavity 108 thicknesses using as few as two selective removalsteps.

Thus, particular embodiments may have multiple, discretely tunedsub-pixel filters that have a material stack in common and that differwith respect to each other only (or at least) in one parameter of theoptical cavity. Various embodiments may use one or more process steps toform respective portions of discretely tuned filters, though the filtersmay be designed to have different resonant responses. Stateddifferently, in various embodiments each super-pixel 102 may be capableof spatially filtering an N number of spectral regions using Nsub-pixels filters. The optical cavity 108 of these N sub-pixels filtersmay be shaped to respective dimensions suitable for desired resonantwavelength using as few removal steps of the optical cavity 108 as thebinary logarithmic function Log₂ N. This enhanced processing feature,which may use one or more processing steps to form respective portionsof discretely tuned sub-pixel filter cavities, may be contrasted withprocessing techniques that form discretely configured filters ordetectors one type at a time in separate process steps.

In particular embodiments, the one or more sub-pixel filters for anygiven super-pixel 102 may be formed adjacent to each other. As shown inFIG. 1C, for example, sub-pixel filters A, B, C, and D of super-pixel102 are separated only by their differing step heights but otherwisehave one continuously joined base. Such a configuration may improvedevice performance by mitigating harmful diffraction that mightotherwise be caused by sub-pixel filters that are spaced apart from eachother and that may present more sidewall surfaces or other featurespossibly contributing to diffraction. Additionally, minimizing thefootprint of each super-pixel 102 by joining the bases of sub-pixelfilters A, B, C, and D may enable multi-color filtration using lessspace. Such a configuration may enable a denser pixel arrangement thatfacilitates higher resolution.

In various embodiments including an array of super-pixels 102, eachsuper-pixel 102 may be joined to one or more adjacently positionedsuper-pixel 102 such that they share a common base in a mannersubstantially similar to that described above with reference tosub-pixel filters A, B, C, and D. That is, in particular embodiments,there is no gap separating adjacent super-pixels 102 from each other,which may provide a number advantages, some of which may be analogous tothose described above with reference to sub-pixel filters A, B, C, D. Inyet other embodiments, there may be a gap separating adjacentsuper-pixels 102 from each other.

Particular embodiments may enable the formation of layers-in-common foreach sub-pixel filter A, B, C, and D. For example, one or more layersdisposed inwardly from optical cavity 108 may function as a firstreflector-in-common for each sub-pixel filter A, B, C, and D. Anotherone or more layers disposed outwardly from optical cavity 108 mayfunction as second reflector-in-common for each sub-pixel filter A, B,C, and D. Although sub-pixels A, B, C, and D may have different stepheights, the second reflector-in-common may be formed outwardly fromeach sub-pixel A, B, C, and D substantially simultaneously. For example,the second reflector-in-common may be formed using molecular beamepitaxy, one or more deposition processes, and/or another processcapable of growing one or more layers outwardly from each sub-pixelfilter A, B, C, and D substantially simultaneously.

FIG. 2 illustrates a cross-section of one example embodiment of asub-pixel filter 200 that may be used to form a portion of thesuper-pixel 102 of FIGS. 1 A through 1C. In this example, sub-pixelfilter 200 includes a stack of multiple layers 202, 204, 206, 208, 210,212, 214, and 216 comprised of germanium (Ge) and zinc sulfide (ZnS).Each layer 202-216 is disposed outwardly from Ge substrate 104. Incertain embodiments, reflective layer 106 may be comprised of Ge and ZnSlayers 202, 204, 206, 208, and 210. In various embodiments, a reflectivecapping layer 110 may be comprised of Ge and ZnS layers 212, 214, and216. Each layer of the sub-pixel filter stack may have any suitabledimensions. In a particular embodiment, Ge layers 202, 206, 210, 212,and 216 may each have a respective average thickness of approximately256.1 nanometers (nm). In certain embodiments, ZnS layers 204, 208, and214 may each have a respective average thickness of approximately 500nm. Although Ge and ZnS is used in the example for substrate 104 and forlayers 202, 204, 206, 208, 210, 212, 214, and 216, any suitablematerials may be used. Additionally, any suitable number of layersincluding, for example, fewer or more layers and/or one or moreinterstitial layers may be used. Although example thicknesses aredisclosed for layers 202-216, any suitable thickness may be used.

An optical cavity 108 or sacrificial cavity is disposed between Gelayers 210 and 212. Although optical cavity 108 is comprised of ZnS inthis example, any suitable dielectric material may be used including,for example, zinc selenide, germanium, silicon, silicon dioxide, and/oralternative dietetic material suitable for optical filters. As discussedpreviously, the thickness of optical cavity 108 may be selected based atleast in part on the desired resonant wavelength response of thecorresponding sub-pixel filter. For example, optical cavity 108 may havean average thickness for sub-pixels A, B, C, and D, of approximately1180 nm, 1050 nm, 920 nm, and 790 nm, respectively. In this manner,super-pixel 102 may be capable of filtering discrete bands, illustratinga four-color example, with each of these bands being applied to a pixelon the IR-FPA, thereby creating a four color super-pixel.

The non-limiting example thicknesses discussed above for the sacrificialoptical cavity 108 of sub-pixels A, B, C, D may, in certain cases, besuitable for a mid-wave infrared (MWIR) transmission window including 3to 5 micrometers (μm). Although the thicknesses used in this examplecorrespond to MWIR, multi-spectral system 100 may be responsive to anysuitable spectral range including, for example, near-infrared (NIR),short-wavelength infrared (SWIR), long-wavelength infrared (LWIR),very-long wave infrared (VLWIR), another region within the infraredspectrum, and/or radiation outside the infrared spectrum. As usedherein, NIR radiation includes a spectral region extending fromapproximately 0.5 to 1 micrometers, SWIR radiation includes a spectralregion extending from approximately 1 to 3 micrometers, MWIR radiationincludes a spectral region extending from approximately 3 to 8micrometers, LWIR radiation includes a spectral region extending fromapproximately 8 to 12 micrometers, and VLWIR radiation includes aspectral region extending from approximately 12 to 30 micrometers.

The components of the systems disclosed herein may be integrated orseparated. Moreover, the functions of the elements and/or layers may beperformed by more, fewer, or other components. For example, particularembodiments may include multiple filtering layers and/or one or morediffraction gratings. As another example, particular embodiments mayinclude one or more super-pixels that each include only two sub-pixelfilters. Particular operations of the systems and apparatuses disclosedherein may be performed using any suitable logic embodied incomputer-readable media. As used in this document, “each” refers to eachmember of a set or each member of a subset of a set.

Although the present disclosure has been described above in connectionwith several embodiments, a myriad of changes, substitutions,variations, alterations, transformations, and modifications may besuggested to one skilled in the art, and it is intended that the presentinvention encompass such changes, substitutions, variations,alterations, transformations, and modifications as fall within thespirit and scope of the appended claims.

To aid the Patent Office, and any readers of any patent issued on thisapplication in interpreting the claims appended hereto, applicants wishto note that they do not intend any of the appended claims to invokeparagraph 6 of 35 U.S.C. §112 as it exists on the date of filing hereofunless the words “means for” or “step for” are explicitly used in theparticular claim.

1. An array of multi-spectral filter elements comprising a plurality ofmulti-spectral filter elements each adapted to respond toelectromagnetic radiation within a plurality of spectral bands, each ofthe plurality of multi-spectral filter elements comprising: a firstsub-filter element adapted to respond to electromagnetic radiationwithin a first one of the plurality of spectral bands, the firstsub-filter element comprising a first optical cavity having an averagethickness of approximately X; a second sub-filter element adapted torespond to electromagnetic radiation within a second one of theplurality of spectral bands, the second sub-filter element comprising asecond optical cavity having an average thickness of approximately X−Y;a third sub-filter element adapted to respond to electromagneticradiation within a third one of the plurality of spectral bands, thethird sub-filter element comprising a third optical cavity having anaverage thickness of approximately X−Z; and a fourth sub-filter elementadapted to respond to electromagnetic radiation within a fourth one ofthe plurality of spectral bands, the fourth sub-filter elementcomprising a fourth optical cavity having an average thickness ofapproximately X−(Y+Z); wherein each of the first optical cavity, secondoptical cavity, third optical cavity, and fourth optical cavity compriserespective portions of a solid dielectric layer.
 2. The array ofmulti-spectral filter elements of claim 1, wherein X is between 5 and5000 nanometers.
 3. The array of multi-spectral filter elements of claim1, further comprising a single reflector disposed outwardly from each ofthe first sub-filter element, the second sub-filter element, the thirdsub-filter element, and the fourth sub-filter element, the singlereflector being a continuous reflector-in-common for each of the firstsub-filter element, the second sub-filter element, the third sub-filterelement, and the fourth sub-filter element.
 4. The array ofmulti-spectral filter elements of claim 1, further comprising a singlereflector disposed inwardly from each of the first sub-filter element,the second sub-filter element, the third sub-filter element, and thefourth sub-filter element, the single reflector being a continuousreflector-in-common for each of the first sub-filter element, the secondsub-filter element, the third sub-filter element, and the fourthsub-filter element.
 5. The array of multi-spectral filter elements ofclaim 1, wherein the first sub-filter element, the second sub-filterelement, the third sub-filter element, and the fourth sub-filter elementare spatially joined together such that the first optical cavity, thesecond optical cavity, the third optical cavity, and the fourth opticalcavity optical cavity collectively comprise an unbroken plane extendingthrough each of the first optical cavity, the second optical cavity, thethird optical cavity, and the fourth optical cavity.
 6. The array ofmulti-spectral filter elements of claim 1, wherein the first, second,third, and fourth ones of the plurality of spectral bands are at leastpartially discrete with respect to each other; and wherein the first,second, third, and fourth ones of the plurality of spectral bands eachcomprise respective spectral wavelength ranges between 1 and 30micrometers.
 7. A method of forming a multi-spectral filter elementcomprising a plurality of sub-filters each adapted to respond toelectromagnetic radiation within respective ones of a plurality ofspectral bands, the method comprising: forming an optical cavity layercomprising at least N spatial regions; and selectively removing at leasta portion of the optical cavity layer in at least one of the N spatialregions, each N spatial region corresponding to a respective one of theplurality sub-filters, the plurality of sub-filters comprising at leastN sub-filters, wherein the selectively removing comprises a number ofremoval steps equal to the binary logarithm function Log₂ N; wherein therespective ones of the plurality of spectral bands are at leastpartially discrete with respect to each other.
 8. The method of claim 7,selectively removing at least a portion of the optical cavity layer inat least one of the N spatial regions further comprises: reducing anaverage thickness of a first one of the at least N−1 number of spatialregions by at least a thickness of X; reducing an average thickness of asecond one of the at least N−1 number of spatial regions by at least athickness of Y; and reducing an average thickness of a third one of theat least N−1 number of spatial regions by at least a thickness of X+Y.9. The method of claim 8, wherein X and Y are each between 1 and 1000nanometers.
 10. The method of claim 7, wherein N is greater than orequal to
 2. 11. The method of claim 7, further comprising forming asingle reflector outwardly from each of the plurality of sub-filters,the single reflector being a continuous reflector-in-common for each ofthe plurality of sub-filters.
 12. The method of claim 7, wherein each ofthe plurality of sub-filters are formed outwardly from a singlereflector, the single reflector being a continuous reflector-in-commonfor each of the plurality of sub-filters.
 13. The method of claim 7,wherein at least two of the at least N number of spatial regions arespatially joined together.
 14. The method of claim 7, wherein therespective ones of the plurality of spectral bands each compriserespective spectral wavelength ranges between 1 and 30 micrometers. 15.A method of forming a multi-spectral filter element comprising aplurality of sub-filters each responsive to electromagnetic radiationwithin respective ones of a plurality of spectral bands, methodcomprising: forming an optical cavity layer; selectively removing anaverage thickness of approximately X from each of a first spatial regionand a second spatial region, wherein the average thickness ofapproximately X removed from the first spatial region is removedsubstantially simultaneously with the removing the average thickness ofapproximately X from the second spatial region; and selectively removingan average thickness of approximately Y from each of the second spatialregion and a third spatial region, wherein the average thickness ofapproximately Y removed from second spatial region is removedsubstantially simultaneously with the removing the average thickness ofapproximately Y from the third spatial region; wherein at least two ofthe first spatial region, the second spatial region, and the thirdspatial region are spatially joined together.
 16. The method of claim15, wherein the respective ones of the plurality of spectral bands areat least partially discrete with respect to each other.
 17. The methodof claim 15, wherein X and Y are each between 1 and 1000 nanometers. 18.The method of claim 15, further comprising forming a single reflectoroutwardly from each of the first spatial region, the second spatialregion, and the third spatial region, the single reflector being acontinuous reflector-in-common for each of the plurality of sub-filters19. The method of claim 15, wherein each of the plurality of sub-filtersare formed outwardly from a single reflector, the single reflector beinga continuous reflector-in-common for each of the plurality ofsub-filters.
 20. The method of claim 15, wherein the respective ones ofthe plurality of spectral bands each comprise respective spectralwavelength ranges between 1 and 30 micrometers.